This invention relates generally to duplex communications systems and more particularly to a method of implementing duplex communications within a single band of radio frequencies.
2. Description of the Prior Art
A duplex communications system is defined as a communications system in which the functions of transmission and reception can be carried on simultaneously at each station in the system. Principal among the known systems are the radio telephone and the cordless telephone.
There are two basic types of land-based mobile radio telephone systems, the mobile telephone service (MTS) and the improved mobile telephone system (IMTS). In each system, the intended radio channel of communication must be selected prior to instigation of that communication. In MTS the user must manually switch between radio channels until a clear (unused-at-the-moment) channel is found. The IMTS provides an automatic scanner system which automatically searches the available radio channels until a clear one is found and then locks onto it. Both systems, however, still suffer from the inconvenience of the push-to-talk mode of operation since the transmit and receive frequencies are usually separated by a fixed frequency spacing so as to provide two-way communications. Users must press a microphone switch to transmit and then release it to receive. This mode of operation can be confusing since it usually means that each communicating user will have to say "over" or some such code word when that communicator is ready to listen to the other party speak. One caller cannot interrupt the other in such a call because, by the very nature of the system, the speaker is not listening when he is speaking.
While the radio telephone is an independent system, the cordless telephone, by contrast, is simply a means of patching into an existing conventional, hard-wired telephone system. That is, radio transmission are used to send a caller's audio signal to the remote unit from the base unit and also to send the audio signal from the operator of the remote unit to the base station and thence to the central telephone system. Thus a telephone conversation may be carried on which is indistinguishable from the same conversation being conducted over a conventional hard-wired telephone system. Voice transmission and reception may thus be accomplished concurrently. This is in contrast to the radio telephone or transceiver in which transmission and reception may not be accomplished concurrently.
The implementations of the cordless telephone range from answer-only cordless handsets to cordless devices which perform all the functions of a hard-wired telephone instrument. True cordless telephones work in conjunction with base stations connected to standard telephone lines. Maximum operating ranges may vary from about 50 feet to more than 500 feet. Most operate on the same general principles, wherein both base units and remote handsets each contain both transmitters and receivers, and whereby the base unit patches the telephone line into a transmitter which transmits to a receiver contained in the remote handset. The operator with the remote handset transmits back to a receiver in the base unit which then routes this received audio information into the hard-wired telephone line.
The cordless telephone system generally consists of two units comprising the base unit or station attached directly to the telephone line through a mating connector, and a wireless remote unit. In presently available conventional models, the base unit transmits audio information from the telephone line to which it is directly attached at a low frequency of about 1.7 MHz. The base unit also contains a receiver which is operated at about 49.8 MHz in the radio spectrum. The base unit is generally equipped with an antenna that is used only for receiving transmissions from the portable unit at this radio frequency. It is necessary to have the base unit connected directly into the AC line in the building so that it may have operating current. There is, however, another very important use made by the base unit of the wiring in the building in which it is located. The output of the 1.7 MHz base unit transmitting system is generally split through a transformer on each side of the AC line and is connected thereto through a blocking capacitor. The building electrical system then becomes a very complicated antenna system for transmission of the 1.7 MHz transmission frequency of the base unit. The remote unit has its transmission near 49.8 MHz, as has been noted, and usually contains a telescoping antenna within its case for transmitting purposes only. For reception of the base signals, the remote unit usually has a loop stick antenna located within its case. The ferrite loop antenna consists of a ferrite core wound with many turns of very small diameter wire which forms a resonance circuit at about 1.7 MHz and serves as the receiving antenna for transmissions from the base station. The reason for using a frequency near 49 MHz for transmission on the remote unit and a frequency of about 1.7 MHz for transmission from the base unit is that if the frequency of the transmitter is very close to that of the receiver in a single unit, then the close proximity of the two antennas involved will cause much of the transmitted signal to be fed into the front end of the receiver located nearby and may cause overload and even damage to circuit components. Thus the separation of about 47 to 50 MHz is adequate to prevent such interference. In this presently available conventional system, the base unit has a retractable whip antenna that is used only for the purpose of receiving transmissions from the remote unit and uses the building wiring as antenna for its own transmissions; while the remote unit has a retractable whip antenna used only for transmitting at its higher transmission frequency, information to the base station and contains also a ferrite loop antenna tuned to the approximately 1.7 MHz transmission frequency from the base station. The frequency separation here is necessary, as has been observed, in order to prevent a transmitted signal from being fed into the front end of a closely located receiver. Ferrite loop antennas, as employed in presently conventional cordless telephones, in addition to being inefficient, also suffer from extreme directivity.
It is interesting to note that the approximately 50 MHz transmission utilized by the remote unit has the capibility to travel a greater distance to be received by the base unit than the 1.7 MHz transmission has the capibility to travel from the base unit. There are several factors involved, one being that the lower frequency has more of a tendency to be absorbed by wiring systems, radio receivers in the home, and many other metallic objects and electronic devices than does the higher frequency transmission. It is also true that the ferrite loop stick antenna is much less efficient than a suitably tuned whip antenna would be. It is also true that transmission distance from the base unit will be determined to a large extent by the size and configuration of the electrical system within the building; the amount of steel and other metals used in the building construction; and whether or not the wiring in the building has been run through some sort of metal conduit. If metal conduit has been used to enclose the wiring, it will be nearly impossible to get any transmission range from the base unit because the antenna (the building wiring system) is now completely encased in a grounded metal shell.
Many different devices have been employed in the attempts to solve the problems presented. Most have either presented new problems or only partially solved the problems presented, or both. Most of the devices tried have thus met special needs as presented by specific problems and have therefore served narrow purposes. These prior art devices, among other disadvantages, have either caused unacceptable attentuation of signals, or unacceptable distortion of these same signals and thus have been unreliable and unpredictable in operation under continued use, and have been very expensive and complicated to manufacture.